This invention relates to methods for testing fluids that have one or more crosslinking agents in them. More particularly, the invention provides a method by which crosslink time, and preferably crosslink temperature as well, can be determined in a manner contemplated to standardize the testing of crosslinked fluids.
In the process of stimulating oil and gas wells, various fluids are used, such as solutions of high molecular weight oil or water soluble polymers. Some of these include particulate material, such as sand. The viscosity of these fluids can be increased by a chemical process known as crosslinking. Crosslinking occurs by introducing into the fluid an ionic agent (the crosslinking agent, or crosslinker), such as transition metal or borate ions. The crosslinker bonds polymer molecules in the primary fluid together, thereby increasing the polymer molecular weight and therefore increasing the viscosity of the resulting mixture. The onset of this phenomenon is monitored and controlled during various types of stimulation treatments to help provide a fluid that will perform as desired after time lapse or temperature change or both.
One critical parameter that needs to be determined for a crosslinked fluid is crosslink time. The elapsed time from crosslinker addition and/or temperature change to the onset of crosslinking is defined as crosslink time. A knowledge of crosslink time is important for a number of reasons. For example, consistent crosslink time measurements throughout a treatment are an indication that operating conditions such as fluid pH, crosslinker addition rate, and crosslinker composition are being controlled in a satisfactory manner. In addition to verification of quality control, crosslink time can have a significant effect on both downhole and surface treating conditions. It is generally desirable to have the onset of crosslinking occur downhole just upstream of the perforations. Increased viscosity from crosslinking helps to transport particulates through the perforations and into the fracture. If the onset of crosslinking occurs shortly after the fluid enters the treating string, increased viscosity from crosslinking will increase surface treating pressure and hence increase the power requirements for Completion of the treatment. Although crosslink time can be shown to be important there has not been a standardized method of determining crosslink time such that crosslink time can be accurately and consistently defined and such that various crosslinked fluids can be compared with each other.
In the prevalent crosslink test method that has been used to determine crosslink time, an operator places a small sample of the fluid in a cup in a heated bath, stirs the sample, visually observes a thickening of the fluid, and performs a subjective "lip" test. This lip test includes removing the cup from the heated bath, tilting the cup until the fluid starts to flow over the lip of the cup, and then righting the cup. If the partially overflamed portion of fluid draws back into the cup when it is righted, the crosslinking is considered to have occurred and the time is noted.
Crosslink temperature is another important parameter that is typically determined with the crosslink time. In the "lip" test method the temperature of the heated water bath is set to provide uniform heating from the starting temperature of the fluid to the well perforation temperature over a time period equal to the fluid transport time down the treatment string. If crosslinking is observed before the perforation temperature is reached, the temperature of the sample at the crosslink time is recorded as the crosslink temperature. If the perforation temperature is reached before the sample crosslinks, the sample is removed from the heated bath and maintained at the perforation temperature for the duration of the test.
To obtain the aforementioned benefits of standardized crosslink testing, there is the need for an automated objective method of testing for crosslinking of a fluid whereby crosslink time and preferably crosslink temperature as well can be objectively determined.